Most of the existing packagings with coaxial connectors are integrated on a printed circuit board (PCB) with surface mount technology (SMT). In SMT, electronics components are assembled by mounting directly onto the surface of the substrate. Multilayer ceramic packages have become a popular choice for microwave integrated circuits (MICs), typically in a quad flat no-leads package (QFN) form. The RF connector is launched into a transmission line that is connected to a ceramic package. Utilizing SMT, the entire components are electrically connected with solder after a high temperature reflow. This approach offers a cheap, repeatable, fast, and, reliable way for connectorized packaging at high volume, but may not be scalable in frequency. Due to the large pads and feature sizes, performance may be limited at frequency above approximately 20 GHz, and the design may become challenging. Most of the new ideas in packaging involve variations in surface mount components packaging and integration with coaxial connectors on PCB.
Another approach for package integration is using chip-on-board (COB) technique. In COB, a bare semiconductor die, which is diced from wafer, is mounted onto a PCB substrate, using an epoxy. Pads of the bare die and the substrate are interconnected by wire bond using, for example, gold or aluminum wires. The die is encapsulated afterwards for protection. COB offers key advantages over SMT, and in some areas the COB has already replaced SMT technology. By eliminating the packaging of die, COB significantly reduces the size and therefore achieves higher density and cost savings. However, long wires may introduce unwanted parasitic that can degrade performances at high frequency. The mismatches in pad sizes between die and PCB can also cause an undesirable high return loss.
A traditional method for coupling a millimeter wave connector to an integrated circuit is to use a hybrid assembly (as shown in FIG. 1) that utilizes an alumina substrate for wire-bonding to the integrated circuit. The assembly 100A of FIG. 1 includes a housing 102 (e.g., metal housing), a radio-frequency (RF) (e.g., millimeter wave) connector 104, alumina substrates 110 and 120, an integrated circuit (IC) 130, and a dielectric layer 132. The millimeter wave connector 104 is coupled (e.g., fixed) to the housing 102 and includes a connector element 106 and a pin 108. The pin 108 is conductively boded (e.g., soldered) to a metallic (e.g., gold) bond pad 112 formed over the substrate 110. The IC 130 is mounted on the dielectric layer 132 and is wire-bonded (through IC bond pads) to the metallic layer 112 using metal (e.g., gold) wires 114. The IC 130 can similarly be wire-bonded to a metallic bond pad 122 (e.g., gold) of the alumina substrate 120, using metal (e.g., gold) wires 124, from there it can be coupled to another millimeter wave connector or another IC (not shown for simplicity).
It is difficult to integrate multiple (e.g., >2) planar layers on alumina, thus, the integration complexity is lower than PCBs. In addition to integration density, alumina substrates are traditionally used with wire-bonded integrated circuits transition from the IC bond pads to alumina bond pads. This approach works well and is economical for low frequency ICs (e.g. up to 45 GHz) with few input/output (IO) pads, but does not scale well for ICs with narrow pitch pads and a large number of IO pads. In addition to scalability, bond wires introduce unwanted parasitic inductances and high characteristic impedance wave guides.